Most dermatology severity indices are continuous scores whose absolute value depends on the extent of disease at baseline, so clinical trials report response as the percentage reduction from the baseline score rather than as a final value. Percentage improvement is calculated as the baseline score minus the follow-up score, divided by the baseline score, multiplied by 100. A response threshold is reached when the improvement equals or exceeds the stated percentage. This convention allows patients who start at very different severities to be compared, and it underlies the endpoints used for regulatory approval and for reimbursement in most jurisdictions, including the Canadian public drug plans.

The Psoriasis Area and Severity Index (Fredriksson and Pettersson 1978) established the pattern. A 75% reduction, PASI 75, became the primary endpoint of the biologic era, and Carlin et al. (2004) showed that PASI 50 is also a clinically meaningful response. The European consensus on treatment goals (Mrowietz et al. 2011) defines treatment success as PASI 75, and treatment failure as less than PASI 50; an intermediate response of PASI 50 to 75 is judged by the Dermatology Life Quality Index, with a score above 5 prompting a change of therapy. PASI 90 and PASI 100 are now the usual benchmarks for the interleukin-17 and interleukin-23 inhibitors.

The Eczema Area and Severity Index (Hanifin et al. 2001) follows the same convention, with EASI-50, EASI-75 and EASI-90 reported alongside a validated Investigator Global Assessment of 0 or 1 with at least a two-point improvement. SCORAD-50 and SCORAD-75 are reported in trials that use the SCORAD index, and percentage responses in the total-body and facial Vitiligo Area Scoring Index (T-VASI50, T-VASI75, F-VASI75 and F-VASI90) were the endpoints of the ruxolitinib cream programme (Rosmarin et al. 2022). NAPSI 50, 75 and 90 are used in nail psoriasis trials, and reductions in the percentage of body surface area affected are reported in the same way.

Hidradenitis suppurativa uses a composite criterion. The Hidradenitis Suppurativa Clinical Response (Kimball et al. 2014) requires a reduction of at least 50% in the combined count of abscesses and inflammatory nodules together with no increase in the number of abscesses and no increase in the number of draining tunnels; HiSCR75 and HiSCR90 raise the reduction to 75% and 90%. Because the HiSCR counts abscesses and nodules equally and ignores existing draining tunnels, Tzellos et al. (2023) derived the IHS4-55, a reduction of at least 55% in the International Hidradenitis Suppurativa Severity Score System (Zouboulis et al. 2017), which weights nodules, abscesses and draining tunnels 1, 2 and 4. This calculator derives both outcomes from the same lesion counts.

Percentage responses have limitations. The same percentage represents a different absolute change at different baselines, a small baseline score makes the threshold easy to reach or impossible to assess, and a response tells nothing about the residual disease. For that reason absolute targets are increasingly recommended alongside percentage responses, such as a PASI of 2 or less or an absolute EASI of 7 or less. This calculator reports both the absolute change and the percentage improvement so that both can be recorded.